1. Field of the Invention
The present invention relates to a semiconductive member used in image forming apparatuses such as the electrophotograph apparatus and static electric recording apparatus of copy machines, laser printers, facsimile machines or office automation equipment that is a combination of these. More particularly, the present invention relates to a semiconductive member used in various procedures in the image forming process of electricity removing members, pressure fixation rolls, copying members, and charging members that accomplish a uniform charging process on the surface of a body being charged, such as a photosensitive body or a dielectric, by pressing against the surface of the body being charged.
2. Description of Related Art
Technology wherein a series of image forming processes are established through control of static electricity is widely known. That is to say, in addition to procedures for directly controlling static electricity, such as charging, exposing, developing and copying, static electricity control (primarily as electricity removal) must also be accomplished in fixation and paper discharge procedures. To this end, a member having a uniform resistance with a volume specific resistance value in the range 10.sup.5 to 10.sup.14 .OMEGA. cm is required.
The members used in the field of image formation are generally composed of a conductive part and a resistance control part, and because the resistance value varies depending on the shape and size of the member, generally, in terms of the volume specific resistance value, a material with 10.sup.8 .OMEGA. cm or less is selected for the conductive part and a material with 10.sup.5 to 10.sup.14 is selected for the resistance control part. The function of the conductive part is to connect to ground and to supply electric current (connect to a power source), while the resistance control unit accomplishes resistance control in accordance with the functions of the other members.
In general, the volume specific resistance value of the material can be adjusted with relative ease for the region of 10.sup.3 .OMEGA. cm or less by using a conductivizing agent such as carbon black or the like, or for the range of 10.sup.14 or greater by using an insulating polymer material. However, even if the apparent volume specific resistance value is in the above-described range of 10.sup.5 to 10.sup.14 as the average value, there are many cases where the uniformity of resistance is poor, and there are many technically difficult points in uniformly adjusting the resistance in the useful region in the field of image formation.
For a member used in image forming apparatus, generally a roll-shaped semiconductive member can be used suitably, but members having the shape of belts, film, blades and brushes and the like are also used.
As one example of these semiconductive members, a charging roll comprised of various types of rubber materials and polymer materials for charging an electrophotograph photosensitive body is disclosed, for example, in Japanese Laid-Open Patent Publication 63-149669. In addition, numerous proposals have been made regarding in what range the electric resistance value of the roll and covering layer can be adjusted to obtain an effective charging amount. Certainly, a sufficient charging amount can be easily obtained using a conductive elastic roll formed and vulcanized after wrapping around a core metal a rubber material in which the electric resistance value has been adjusted through kneading this with a conductivizing agent such as carbon black or the like.
In addition, in contact charging members using a roll- or plate-shaped conductive elastic body, it is commonly known that the rubber hardness of the elastic body in the charging member should be made JIS A 40.degree. or less in order to obtain uniform contact with the body being charged such as a photosensitive body. It is also known that a compound in which various softening agents are mixed into low hardness pliable silicone rubber and various hydrocarbon-based rubbers is used as a charging member with low hardness.
However, these rubber materials all contain migrating, low molecular weight substances. Consequently, when the charging member makes contact for a long time with a photosensitive body, for example, the problem arises that image defects such as streaks or white voids are created because of substances that have migrated to the photosensitive body. In addition, when the charging member is applied to a copy machine, the back surface of the copy material such as paper can be stained.
That is to say, in conventional semiconductive members, in particular charging members, a first problem is that when the hardness of the contact charging member is kept low and a low hardness charging member is used, it is difficult to avoid migration of the softening agent and the like to the body being charged.
A second problem is that when a roll made of a material with poor permanent warping characteristics such as SBR (styrene butadiene rubber), for example, out of the hydrocarbon rubbers, is left for a long period of time in contact with the body being charged, the roll deforms and charging cannot be accomplished properly.
As a third problem, in cases where there are pinholes in the surface of the body being charged and the conductive drum made of aluminum of the like is exposed, when the charging process is accomplished using the charging roll, a large electric current flows suddenly to the pinhole area, and the charging roll instantly ruptures. Simultaneously, the area of contact between the body being charged and the roll member suffers poor charging, causing straight line streaks to appear on the copy paper.
Furthermore, as a fourth problem, during continuous operation in a low temperature, low humidity environment, when continuous operation is executed while accomplishing exposure electricity removing using an LED, the electrical resistance rises dramatically, causing poor charging to occur.
A large number of countermeasures have been proposed from before to resolve the above-described problems.
To cope with the first problem, Japanese Laid-Open Patent Publication 2-311867, for example, proposes a conductive roll covered with a softening agent migration preventing layer containing a conductive agent with the primary component being N-methoxymethyl nylon covering the conductive elastic layer in which ketchen black and a naphthene-based oil, as a softening agent, are mixed into a hydrocarbon synthetic rubber. However, in this roll, the migration preventing effect is insufficient, and the thickness of the resistance adjusting layer on the migration preventing layer needs to be at least 50 .mu.m. Consequently, problems arise that the hardness of the roll rises and the productivity during film formation is extremely low.
In Japanese Laid-Open Patent Publication 3-249777, a charging member is proposed which prevents the bleeding phenomenon known as roll trace by covering the elastic layer, in which carbon black is dispersed in a silicone rubber, with a surface layer in which the primary component is N-methoxymethyl nylon. However, in this charging member, while it is possible to make the thickness of the surface layer thinner, problems arise that the environment dependence of the electric resistance for the N-methoxymethyl nylon is large, and charging unevenness and leak defects occur easily.
In Japanese Laid-Open Patent Publication 4-120564, a charging material is proposed in which a lower resistance layer, in which carbon black with an oil absorption of 80 ml/100 g or less is dispersed in silicone rubber, is covered by an upper resistance layer in which conductive particles are dispersed in resin or rubber, and polyurethane is disclosed as the material for comprising the upper layer. However, in this charging member, problems arise that the uniformity of the electrical resistance and the environmental stability are poor, so that when the charging member is made to contact the surface of the photosensitive body for one month or more, tacking occurs with the photosensitive body. In addition, it is noted in this publication that good adhesion can be obtained if the upper layer and lower layer are adhered using a silane coupling agent, so the adhesion between the silicone rubber layer and the upper layer is poor and a special processing procedure is necessary in the adhesion process, in addition to peeling off of the film occurring over long-term use.
Japanese Laid-Open Patent Publication 6-25431 proposes an electrophotograph silicone rubber roller in which a silicone rubber compound, having a total concentration of low grade polymer siloxane and nonfunctional silicone oil of 5000 ppm or less, is formed into a roll shape and then undergoes a vacuum heating process to remove the migrating components such as the above-described siloxane and oil, and a method of producing such. However, this method has extremely low productivity because a procedure to remove the migrating components is necessary before or after formation, creating the problem that the bleeding prevention effect is not necessarily sufficient.
Concerning the second problem, it is commonly known from various types of documents that a good effect with no deformation of the nip part can be obtained if the above-described SBR and the like is avoided and silicone rubber and fluoro-silicone rubber and the like are used.
Concerning the third problem, generally a resistance control layer is provided which has a volume specific resistance value of 10.sup.6 to 10.sup.10 .OMEGA. cm, or more preferably 10.sup.7 to 10.sup.9 .OMEGA. cm. For example, in Japanese Laid-Open Patent Publication 1-142569, a conductive roll is proposed in which a resistance layer composed of epichlorohydrin-ethylene oxide copolymer rubber (ECO) with a volume specific resistance x thickness of 10.sup.5 to 10.sup.7 .OMEGA. cm.sup.2 covers the conductive elastic layer. In other words, charging is possible using ECO, which has the lowest electrical resistance among general polymer materials, while insulation breakdown is prevented.
In Japanese Laid-Open Patent Publication 2-198470, a semiconductive roll is proposed containing perchlorate in the polymerizing compound having an ether coupling in repeated units. In this semiconductive roll, the perchlorate is configured in the oxygen atoms giving the ether coupling, and consequently the rise in the resistance is small even in low humidity environments. In addition, in Japanese Laid-Open Patent Publication 7-28301, a conductive roll is proposed in which a protective layer is provided on top of a base layer with a volume specific resistance of 10.sup.6 to 10.sup.8 .OMEGA. cm composed of an ion conductive foam to which quaternary ammonium and perchlorate have been added. In this conductive roll, the base layer itself has intermediate resistance, and consequently the permissible width of voltage impression is wide and image defects are prevented.
Furthermore, in a roll having carbon black merely dispersed in a rubber material and a polymer material, there are often problems with uniformity in resistance, and the phenomenon known as charge unevenness occurs. Accordingly, the role of the resistance control layer is large in the sense of securing uniformity in the resistance.
In Japanese Laid-Open Patent Publication 4-14070, a new proposal is made for creating a resin layer of a polyester amide composed of dibasic carboxylic acid, diol and .omega.-amino carboxylic acid, but no reduction to practice has been achieved yet.
In recent years, graft copolymer resins of polyether amide and polyester amide and the like and known as "electricity control resins" have been marketed (e.g., see Umeda, Matsuzawa, Plastics Age, 40(4), 104(1994)). These resins have a permanent static electricity preventing action when blended with resins having a high insulating ability with a tendency to cause adhesion of dust through static electricity, and consequently are widely used for example in the housings of office automation equipment. The volume specific resistance values of these resins are on the order of 10.sup.9 .OMEGA. cm, in a range making the resins usable as materials to compose the above-described resistance control layer.
However, when the above-described materials are used as the materials composing the resistance control layer, it has become clear that the following problems arise. That is to say, in conductive rolls having as the resistance layer ECO that has actually been marketed, the environmental dependence of the electrical resistance is large, the charging capability differs depending on the changes in the environment, the charging voltage is low in low temperature and low humidity environments and pinhole leaks occur easily in high temperature and high humidity environments. In particular, the resistance control layer cannot achieve the original performance in low temperature, low humidity environments.
In addition, with a conductive roll of the ion conduction type, the electrical resistance of the roll drops through the coordination of the ion conductivizing agent such as perchlorate or the like into the polymer material. Consequently, the initial charging poorness in low temperature, low humidity environments is resolved, but another serious, major flaw arises during continuous operation. That is to say, even in the ion conduction type, the resistance of the roll gradually increases after repeated usage in low temperature, low humidity environments as mentioned as the above-described fourth problem, and eventually poor charging occurs.
Furthermore, with a charging roll where the above-described electricity control resin is used as the material comprising the resistance control layer, there is efficacy on the point of preventing insulation breakdown, but perhaps because the resistance is too high, poor charging of the photosensitive body occurs in low temperature, low humidity environments.
However, in the copying procedure, generally a copying member with higher electrical resistance than in the charging procedure is used. This copying member also uses a rubber roll itself such as foam polyurethane with a volume specific resistance value in the range of 10.sup.10 to 10.sup.14 .OMEGA. cm, or such a roll with the surface thereof covered with a protective layer. However, even in this kind of copy roll the problem of resistance control is still not resolved. That is to say, when the attempt to control the resistance of the roll is made by dispersing particle-shaped conductive powder such as carbon black or the like, the variance in the surface resistance is large and copy unevenness occurs which is not acceptable in practice, perhaps because the structure between the conductive particles is solid, making dispersion difficult.
In addition, the electricity control resin in the market is preferable from the standpoint of the volume specific resistance value, but the hardness is too high making it impossible to form a suitable nip, and the hardness is in a range that makes usage impossible as a material for forming an elastic layer. This electricity control resin can be used as a material to cover the conductive foam urethane rubber, for example, but the environmental dependence of the resistance is high and as a result the environmental dependence of the copying ease is high, which is not acceptable in practice.
In the fixation procedure, there are many cases where the fixation apparatus is composed from a heating roll that heats the toner image on the copy paper and a pressure roll which is pressed against this. In this kind of fixation apparatus, when the surface protection layer of the pressure roll is insulated, static electricity accumulates and a phenomenon occurs wherein the toner adheres to the heating roll.
In order to prevent this, there are many cases where a covering layer is provided that gives a suitable conductivity. When the environment dependence of the electrical resistance of this covering layer is large, offsetting of the toner arises. The above-described electricity control resin is preferable from the standpoint of the volume specific resistance value as a pressing roll, but similar to the case of the above-described copy roll, the hardness of the elastic layer is in a range that makes usage impossible. For example, although this can be used as a covering material for conductive foam urethane rubber rolls, the environmental dependence of the resistance is high and as a result offsetting of the toner occurs because of fluctuations in the environment, which is not acceptable in practice.
Furthermore, a material suitable for accomplishing removal of electricity from the charged copy paper and the surface of the body being charged still does not exist for the same reasons as discussed above.
Thus, a material that can satisfy completely the resistance control of the member used in the various procedures of an electrophotograph apparatus and a static electric recording apparatus or the like currently does not exist.